Resin piston for master cylinder in brake system, master cylinder, die for molding resin piston for master cylinder, and manufacturing method of resin piston for master cylinder

Abstract

An upper die and a lower die are combined. A core for molding a concavity and another core are combined with the combined upper die and the lower die at respective prescribed positions. A core for through-hole molding is constructed by the junction of cores that are severally formed with the upper die and the lower die integrally on the extension line of the central axis of the core for molding a concavity. A through-hole is molded with the core for through-hole molding. A concavity in which a valve mechanism is equipped and a communicating hole through which the concavity communicates with the through-hole are molded with the core for molding a concavity. The core for through-hole molding includes an impact hole, in which a tip of the core for molding a concavity is impacted.

What is claimed is:  
     
       1. A resin piston for a master cylinder comprising: a through-hole through which a stopper pin being a component of a valve mechanism of said master cylinder is inserted, a concavity in which said valve mechanism is fixed by being inserted therein, and a communicating hole communicating with said through-hole from said concavity, said resin piston for a master cylinder being molded by injection of a resin material; and 
       a burr generated by a flow of said resin material into a gap between a combined molding die and a core in a process of molding of said resin piston by injection molding, said burr projecting out from said communicating hole into the through-hole;  
       wherein a groove is provided in a portion of an inner wall surface of said through-hole, the groove facing away from said communicating hole, the groove being in the shape of a flat surface oriented in a direction substantially perpendicular to the longitudinal direction in which said stopper pin travels in operation of said master cylinder, a width of said groove being narrower than a width of said through-hole and wider than a diameter of said stopper pin.  
     
     
       2. A master cylinder equipped with a resin piston for said master cylinder, said resin piston for a master cylinder comprising: 
       a through-hole through which a stopper pin being a component of a valve mechanism of said master cylinder is inserted, a concavity in which said valve mechanism is fixed by being inserted therein, and a communicating hole communicating with said through-hole from said concavity, said resin piston for a master cylinder being molded by injection of a resin material;  
       a burr generated by a flow of said resin material into a gap between a combined molding die and a core in a process of molding of said resin piston for a master cylinder by injection molding, said burr projecting out from said communicating hole into the through-hole, wherein said through-hole is molded with a core for through-hole molding; said concavity and said communicating hole are molded with a core for molding a concavity; and  
       a groove is provided in a portion of an inner wall surface of said through-hole, the groove facing away from said communicating hole, the groove being in the shape of a flat surface oriented in a direction substantially perpendicular to the longitudinal direction in which said stopper pin travels in operation of said master cylinder, a width of said groove being narrower than a width of said through-hole and wider than a diameter of said stopper pin.